The Church Rock uranium mill spill occurred in the U.S. state of New Mexico on July 16, 1979, when United Nuclear Corporation's tailings disposal pond at its uranium mill in Church Rock breached its dam. The spill remains the largest release of radioactive material in U.S. history, having released more radioactivity than the Three Mile Island accident four months earlier.
The Church Rock uranium mill, which operated from June 1977 to May 1982, was located on privately owned land about northeast of Gallup, New Mexico, bordered to the north and southwest by Navajo Nation Tribal Trust lands. The milling of uranium ore there produced an acidic slurry of ground waste rock and fluid (tailings) that was pumped to the tailings disposal area.
The breach released more than of solid radioactive mill waste and of acidic, radioactive tailings solution into the Puerco River through Pipeline Arroyo. An estimated of uranium and 46 curies of alpha contaminants traveled downstreamLauri Wirt (1994), "Radioactivity in the Environment: A Case Study of the Puerco and Little Colorado River Basins, Arizona and New Mexico". Tucson, AZ: U.S. Geological Survey Water Investigations Report 94-4192. https://pubs.usgs.gov/wri/1994/4192/report.pdf to Navajo County, Arizona, and onto the Navajo Nation. In addition to being radioactive and acidic, the spill contained toxic metals and sulfates. The spill contaminated groundwater and rendered the Puerco unusable to local residents, mostly Navajo peoples who used the river's water for drinking, irrigation, and livestock. They were not warned for days of the toxic dangers from the spill.
The governor of New Mexico at the time, Bruce King, refused the Navajo Nation's request that the site be declared a federal disaster area, limiting the aid affected residents could receive. The nuclear contamination event received less media coverage than that of Three Mile Island, possibly because it occurred in a very rural area not served by major media. The spill also happened in Native American country, among a community who reportedly did not have their concerns addressed by medical authorities.
In 2003, the Church Rock Chapter of the Navajo Nation began the Church Rock Uranium Monitoring Project to assess environmental impacts of abandoned uranium mines; it found significant radiation from both natural and mining sources in the area. As of 2016, the EPA National Priorities List included the Church Rock tailings storage site, where "groundwater migration is not under control".
Dam failure At around 5:30 am on July 16, 1979, a previously identified crack opened into a in the south cell of United Nuclear Corporation's Church Rock temporary uranium mill tailings disposal pond, and of solid radioactive mill waste and about of acidic, radioactive tailings solution flowed into Pipeline Arroyo, a tributary of the Puerco River. Warnings of an impending spill had been ignored by the state and by United Nuclear Corporation. Though the uranium mill only bordered the Navajo Nation, the tailings spilled onto the Navajo Nation as they flowed down the Puerco River.
The tailings solution had a pH of 1.2 and a gross alpha particle activity of per liter. In addition to radioactive uranium, thorium, radium, and polonium, it also contained various other metals, including cadmium, aluminium, magnesium, manganese, molybdenum, nickel, selenium, sodium, vanadium, zinc, iron, and lead, and also high concentrations of sulfates. The contaminated water from the Church Rock spill traveled downstream, through Gallup, New Mexico, and reached as far as Navajo County, Arizona. The flood backed up sewers, affected nearby aquifers, and left stagnating, contaminated pools on the riverside.
As the highly acidic spill traveled downstream, alkaline soils and clays neutralized the acid and adsorbed many of the contaminants. The contaminated sediments were gradually dispersed by the river and diluted by "clean" sediment. In parts of the river system with higher concentrations of contaminants, yellow salt crystals precipitated onto the arroyo bed. These salts, containing metals and radionuclides, were washed away during subsequent rainstorms. Approximately one month after the spill, the Puerco River had regained normal levels of salinity, acidity, and radioactivity at low flow levels, with contaminants being detectable only after heavy rains. The EPA reported no long-term effects of the spill, but noted that contaminant levels from uranium mine effluents and natural sources were "environmentally significant".
Response At 6:00 am, a United Nuclear Corporation employee noticed the breach and suspended further discharge of tailings solution to the holding pond. By 8:00, a temporary dike had stopped the flow of residual tailings solution.
Several days after the spill, the Indian Health Service and the Environmental Improvement Division of New Mexico warned local residents by radio and with signs written in English not to drink from, water livestock at, or enter the Puerco River. However, many Navajo people in the area speak only Diné Bizaad, an Athabaskan language spoken by 150,000 people on the Navajo Nation.
The states of Arizona and New Mexico did not immediately make their residents aware of the dangers of radiation. Not until a few days after the spill were United Nuclear Corporation employees dispatched to warn Navajo-speaking residents downstream in accordance with a state contingency plan.US Congress, House Committee on Interior and Insular Affairs, Subcommittee on Energy and the Environment. Mill Tailings Dam Break at Church Rock, New Mexico, 96th Cong, 1st Sess (October 22, 1979):19–24.Kathie Saltzstein, "Navajos Ask $12.5 Million in UNC Suits", Gallup Independent, August 14, 1980 The Navajo Nation asked the New Mexico governor Bruce King to request disaster assistance from the US government and have the site declared a disaster area, but he refused, an action that limited disaster relief assistance to the Navajo Nation.
Although the New Mexico Environmental Improvement Division said the spill's "short-term and long-term impacts on people and the environment were quite limited", ponds of uranium-contaminated water lined the Puerco River and seeped into wells. United Nuclear denied claims that the spill caused livestock deaths, even though harm to the Navajo economy (which was dependent on the sale of mutton) was claimed. The company issued this statement through an attorney: "We just don't know of any substance to those claims. Some people aren't going to be satisfied no matter how thoroughly you show it." Navajo Tribal Council's vice president Frank Paul said of the worst spill in US history: "Somehow, United Nuclear Corporation was permitted to locate a tailings pond and a dam on an unstable geologic formation. Somehow, UNC was allowed to design an unsafe tailings dam not in conformance to its own design criteria. Somehow, UNC was permitted to inadequately deal with warning cracks that had appeared over two years prior to the date the dam failed. Somehow, UNC was permitted to continue a temporary dam for six months beyond its design life. Somehow, UNC was permitted to have a tailings dam without either an adequate contingency plan or sufficient men and material in place to deal with a spill. Somehow, UNC was permitted to deal with the spill by doing almost nothing."United States. Congress. House. Committee on Interior and Insular Affairs. Subcommittee on Energy and the Environment. Mill Tailings Dam Break at Church Rock, New Mexico: Oversight Hearing Before the Subcommittee On Energy And the Environment of the Committee On Interior And Insular Affairs, House of Representatives, Ninety-sixth Congress, First Session ... Hearing Held In Washington, D.C., October 22, 1979. Washington: U.S. Govt. Print. Off., 1980.
Under the "agreement state" legislative framework of the Uranium Mill Tailings Radiation Control Act, the Nuclear Regulatory Commission left New Mexico to handle the dam failure until October 12, 1979, when it was notified that the state would permit the uranium mill to resume operation that week. The NRC then suspended United Nuclear's operating license until it could be determined that the embankment was stable. After fewer than four months of downtime following the dam failure, the mill resumed operations on November 2, 1979. This resumption further contaminated the groundwater and resulted in the mill site's placement on the EPA's National Priorities List in 1983. United Nuclear made a $525,000 out-of-court settlement with the Navajo Nation a year after the spill.
The spill released more radioactivity than the Three Mile Island accident. The spill has been called "the largest radioactive accident in U.S. history", but the Nuclear Regulatory Commission has said that this is "an overstatement" and that "there have been a number of other events that have been more significant in terms of radiological impact. The event was more significant from an environmental perspective than from a human one." Nevertheless, the incident remains the "largest single release" of radioactive materials into the environment in US history as of March 2025.
Causes
The dam formed the southern wall of one of the mill's three holding ponds, which were used to evaporate tailings solution until the remaining solid waste could be buried. From 1967 to 1982, the mill produced an average of of tailings every day, for a total of .. https://www.epa.gov/sites/default/files/2017-12/documents/engineering_evaluation_-_cost_analysis_20090530.pdf The high embankment was constructed on a deposit of collapsible clayey, silty sand, deep. United Nuclear used a new design, recommended by the Nuclear Regulatory Commission, that used earth rather than tailings themselves as building material. The holding pond was not lined, a violation of the Uranium Mill Tailings Radiation Control Act of 1978. This allowed tailings solution to seep into the ground, weakening the foundation of the dam and contaminating the groundwater.
Horizontal and vertical cracks formed along the southern part of the embankment, allowing the acidic tailings solution to penetrate and weaken the embankment. A sand beach was constructed to protect the face of the embankment from the tailings solution, but it was not properly maintained. The liquid in the holding pond eventually rose higher than the dam's designed limit, past the point where the sand beach could protect the dam.Wasserman, Harry and Norman Solomon, Killing Our Own: The Disaster of America's Experience with Atomic Radiation, New York: Dell Publishing Co, 1980. The United States Army Corps of Engineers concluded in its report to Governor Bruce King of New Mexico that the principal cause of failure was differential settlement of the foundation beneath the dam wall, and the report commissioned by the Nuclear Regulatory Commission corroborated this conclusion. Critical variations in tailings pond operation practice from approved procedures contributed further to the dam failure. United Nuclear's chief operating officer, J. David Hann, blamed the failure of dam on the pointed shape of the bedrock beneath the embankment, which he said acted as a fulcrum and weakened the dam.